1. Field of the Invention
The present invention relates to a method of producing slices, such as semiconductor wafers, by severing a rod-like ingot of a semiconductor material without use of any cutting means such as an inner peripheral blade slicer or a wire saw.
2. Description of the Prior Art
Among methods of obtaining semiconductor wafers or like slices from a rod-like ingot of silicon single crystal, are one as disclosed in Japanese Laid-Open Patent Publication No. 3-142928 (herein referred to as first prior art) in which the ingot is sliced into wafers with a thin doughnut-like inner peripheral blade with fine grindstone particles secured to the inner periphery, and one in which a wire saw is used for slicing the ingot.
As the method using the wire saw, various processes have been proposed and adapted for general use. One such method is disclosed in Japanese Laid-Open Patent Publication No. 3-239507 (herein referred to second prior art).
In this method of slicing using the wire saw, grindstone particles are added to a cutting solution for use as free grindstone particles to slice the ingot into wafers with the pH of the process solution set to 7.5 to 9.0.
In the above first and second prior arts, however, a cutting margin is necessary. When slicing the ingot into wafers, the cutting margin is wasted, thus increasing the consumption of the material (i.e., ingot) and causing a cost increase.
Particularly, with the inner peripheral blade slicer pertaining to the first prior art, the wafers obtained by slicing have great warping or denatured layer. This is improved in the case of the wire saw pertaining to the second prior art. Nevertheless, the occurence of warping is inevitable. In addition, because of the use of grindstone particles, the wafers obtained by slicing have denatured layer which is generated by the processing. It is necessary, however, that the product has high degree of flatness and is free from strain. Therefore, the denatured layer has to be removed in a fast-treatment.
The slicing method using the inner peripheral blade as in the first prior art further has a problem that the operating environment is deteriorated by scattering of the liquid coolant used in the slicing operation. The second prior art using the wire saw has like scattering problem because of the use of a cutting solution containing free grindstone particles.
Aside from the processes of direct slicing of ingot using a blade as in the first and second prior arts noted above, a side pressure slicing process (third prior art) has recently been developed (as disclosed in Journal of the Precision Machine Engineer's association of Japan, Vol. 60, No. 2, 1994), in which hard and fragile materials, typically glass or ceramics, are hydraulically sliced with an oil.
Specifically, in the side pressure slicing process, use is made of an internal stress generated in a work by applying pressure side-wise of the work. The principles underlying this process will be described with reference to FIG. 5. Reference numeral 50 designates a cylindrical ingot (i.e., work) to be sliced, and reference numeral 51 designates a pressure vessel, in which the ingot 50 is supported for axial movement by O-rings 52 fitted inside the vessel. A working fluid feed port 51a externally extends into the vessel 51 with the ingot 50 disposed therein.
In this apparatus, pressure P is applied to the inside of the pressure vessel 51 from the working fluid feed port 51a in a state that the leakage of the working fluid is prevented by the two O-rings 52. The ingot 50 receives the applied side pressure P between the two O-rings 52 while its ends are free. Upon reaching of a certain pressure value by the side pressure P, severing takes place momentarily in a section of the ingot 50.
This side pressure slicing process can substantially preclude the problems in the slicing processes using the slicing apparatuses pertaining to the first and second prior arts. However, the ingot 50 is severed at its lowest internal stress position between the O-rings 52 in the pressure vessel 51. This means that it is impossible to determine the position, at which the severing takes place. This is unsuitable for the production of slices having a uniform thickness.